The present invention relates to lead-free alloys for use in soldering. More particularly, the present invention relates to a lead-free solder composition comprising tin, zinc, indium and bismuth.
Different solder compositions have unique characteristics which make them suitable for particular applications. Two characteristics of a solder which are important to its use are melting temperature and melting range.
Melting temperature is one of the most important characteristics of a solder. The solder chosen for a particular use should have a low enough melting temperature that the melted solder does not damage any temperature-sensitive components that are to be joined. However, the melting temperature should also be high enough that the joint formed will not be affected by the operating temperature of the device or by subsequent soldering operations. In modern electronic applications, the temperature sensitivity of microelectronic components requires the use of solders at relatively low temperatures. In comparison, solders for joining and sealing pipes in plumbing operations are generally applied at much higher working temperatures because the components are not so temperature sensitive.
Another important characteristic of a solder is melting range. Pure elemental metals have a melting point. Most alloys, however, with the exception of eutectic compositions, melt over a range of temperatures. The alloy begins to melt at a temperature called the solidus but is not completely liquid until it reaches a higher temperature called the liquidus. The range between the solidus and the liquidus is referred to as the phase transformation range or "pasty" range. At temperatures within the "pasty" range, the alloy contains a mixture of solid and liquid phases containing different metal compositions. The solid phase contains higher melting point components and the liquid phase lower melting point components. Separation of the two components, called liquation, can alter the chemical composition of the alloy and the physical characteristics of the resulting joint.
Liquation can be particularly problematic in automated soldering operations in which components, such as circuit boards, are transported by a conveyer belt through the soldering apparatus. After the solder has been applied by a process such as a wave soldering, the conveyor carries the components into a cooling zone. As the soldered joints cool, the solder solidifies. If a solder with a large "pasty" range is used, then parts of the soldered joint will begin to solidify while some of the solder remains liquid. Vibration from the conveyor belt will then tend to separate the two metal phases. The vibration and liquation may disrupt the crystallization of the solder. The disrupted joint may be physically weakened and conduct electricity poorly or not at all resulting in a circuit which is prone to failure or completely non-functional. In such applications, it is much preferable to use a eutectic solder or a solder with a small "pasty" range.
Solders with small "pasty" ranges are also important in certain "step-soldering" operations where components are added to a device sequentially. These operations are also dependent upon solders with specific melting temperatures. In step soldering, the first components are joined using a relatively high melting temperature solder. When later components are joined, a lower melting temperature solder is used so that the earlier-soldered joints are not affected by the soldering operation. Further components may then be added using solder with an even lower melting temperature. The availability of solders with different melting points is critical to such step-soldering processes. It is also important, if several soldering steps are to be performed, for the melting ranges of the solders to be small.
Several solders are in common use in automated soldering operations. SN63 composed of 63% tin and 37% lead, is a eutectic alloy which melts precisely at 183.degree. C. SN62 composed of 62% tin, 2% silver and 36% lead is a near eutectic alloy with a melting temperature of 179.degree. C. SN60 composed of 60% tin and 40% lead has a solidus temperature of 183.degree. C. and a liquidus temperature of 188.degree. C. giving it a "pasty" range of 5.degree. C. These solders are defined in Federal Specification QQ-S-571E Interim Amendment 5 (ER) 28 December 1989 (hereinafter "Federal Specification QQ-S-571E"). These solders have good characteristics for automated soldering. However, they suffer from the disadvantage that they contain lead.
Lead is known to have toxic effects. For this reason, rigorous limitations have been imposed upon the use of lead and lead-containing compositions. These limitations upon the use of lead-containing solders are most stringent in connection with plumbing where, until recently, the most popular plumbing solder was Sn50Pb50 which comprises 50% lead and 50% tin. Recent federal legislation banned the use of lead-containing solders in potable water systems forcing plumbers to stop using Sn50Pb50 and turn to lead-free solders.
Although plumbing is the most vivid example, other uses of lead-containing solders are also regulated. The United States Occupational Safety and Health Administration ("OSHA") has established a complex and extensive lead standard which regulates the permissible lead concentration in the air in the work place. In situations that result in high levels of lead in the air, OSHA regulations have strict requirements for minimizing employee exposure. Although most situations in which lead-containing solders are used do not produce lead concentrations high enough to trigger the OSHA standards, it is possible that stricter limitations upon the use of lead in solder might be imposed. Even in the absence of such regulations, reducing employee exposure to lead is still desirable. It would, therefore, be desirable to reduce the dependence upon lead-containing solders for certain applications by providing lead-free alternative solders.
It would also be desirable to provide lead-free solder compositions with relatively low melting temperatures suited to the assembly of electronic components.
It would further be desirable to provide lead-free solder compositions with relatively small "pasty" ranges suitable for use in automated soldering operations.
It would also be desirable to provide lead-free solder compositions which can replace currently used lead-containing solders such as SN63, SN62 and SN40.